Changing DPI doesn't work

Created by: RussianNeuroMancer

At least on Kubuntu 13.04 (Solaar installed from PPA) with Performance MX.

Traceback (most recent call last):
  File "/usr/lib/python2.7/threading.py", line 552, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.7/threading.py", line 505, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/opt/extras.ubuntu.com/solaar/share/solaar/lib/solaar/ui/config_panel.py", line 36, in _process_apply_queue
    value = setting.write(value)
  File "/opt/extras.ubuntu.com/solaar/share/solaar/lib/logitech/unifying_receiver/settings.py", line 58, in write
    data_bytes = self._validator.prepare_write(value)
  File "/opt/extras.ubuntu.com/solaar/share/solaar/lib/logitech/unifying_receiver/settings.py", line 164, in prepare_write
    raise ValueError("invalid choice " + repr(value))
ValueError: invalid choice ' 1500'